Superconductor structures have been utilized previously to develop light detectors which exhibit high sensitivity, e.g., on the order of 10.sup.-3 to 10.sup.3 volts/watt, fast response time on the order of a nanosecond, and a wide working wavelength range which spans from the ultraviolet to the far infrared.
Although superconductivity was initially discovered by H. K. Onnes in 1911, it was only recently that superconductive materials with high transition temperatures were discovered. The initial superconductors required sophisticated cryogenic equipment and needed to be chilled with coolants such as liquid helium. These stringent requirements limited the number of applications for the low temperature superconductors. On the other hand, with the discovery in 1986 of cuprate materials having transition temperatures as high as 127.degree. K., many new superconductor applications may be identified and commercialized in the near future. Most important in this regard is the fact that the high T.sub.c superconductors can be chilled with liquid nitrogen which has a boiling temperature of 77.5.degree. K. Liquid nitrogen is relatively inexpensive and easy to use. Thus, high T.sub.c commercial applications are starting now to emerge, such as cryogenic fluid-level monitors and SQUIDS.
Currently, virtually all light sensors are made from solid state materials, such as semiconductors, metals and superconductors. An example of this type of light sensor is shown in Nishino et al, U.S. Pat. No. 5,057,485 which employs a photoconductive semiconductor disposed in intimate contact with a superconductor junction. The overlying semiconductor absorbs the light, but only in a broad, non-discriminating on bandwidth, i.e., a broad range of frequencies which means it is not easily tuneable to specific wavelengths of light. Once light is absorbed by the photoconductor in the Nishino system, charge carriers with relatively long lifetimes are generated in the semiconductor, and the effect persists after the light is switched off, i.e., it is not suitable for fast switching. Further, Nishino's system involves use of inorganic solid state photoconductive semiconductors such as CdS and CdSe. These materials are highly insoluble, refractory materials that are deposited onto the superconductor film using sophisticated sputtering or plasma deposition equipment and techniques. In the Nishino device, light exposure on the photoconductor causes the photogeneration of free charge carriers in the proximal semiconductor which influence the transport properties of the underlying superconductor. Such process leads to a weakening of superconductivity that can be sensed electronically.
Thin film superconductors have also been used as a photodetecting device. Murakami et al (U.S. Pat. No. 4,578,691) describes a photo-detecting device having a Josephson junction that is capable of detecting an optical signal at high speed. The device comprises a polycrystalline superconductor film formed over an insulating substrate. The polycrystalline superconducting film is necessary because light must penetrate into the semiconductor film in order to create quasiparticles in the irradiated superconductor. These quasiparticles change the superconductor energy gap, which in turn causes an output voltage change in the current-biased Josephson junctions. Murakami does not employ any dye layer for any purpose for wavelength discrimination.
The discovery of high temperature superconductors have also been used to investigate the photo-response of a superconductor to incident light. Forrester et al (Appl. Phys. Lett. 53 (14), Oct. 3, 1988) describes a high temperature epitaxial yttrium-barium-copper oxide (YBCO) superconducting film as an optical detector. Forrester does not employ a dye layer for any purpose. Forrester's results show that the response of the superconductor to polychromic light is bolometric. In other words, the polychromic light is absorbed by the superconductor and heats it to above its critical temperature To, thereby increasing its DC resistance.
Light can either be absorbed, transmitted, or reflected by a material. Generally, the intensity of reflected light from most materials has little variation over the visible region. Therefore, the absorbance and transmission spectra are typically near mirror-images of one another. This is shown for the dye, zinc octaethylporphyrin, in FIGS. 12 and 13. As is shown in FIG. 12, zinc octaethylporphyrin absorbs visible light very effectively around 380 nm with weaker absorption bands around 530 nm and 565 nm. At these wavelengths, incoming light is captured by the dye, and cannot pass through the sample. The transmission spectrum for the sample is shown in FIG. 2. Here, several broad transparent regions are present between 400 nm and 520 nm as well as at wavelengths greater than 575 nm. In these regions the dye layer transmits 90 to 95% of the incident light.
A wavelength selective filter transmits electromagnetic radiation in a narrow region around a specific wavelength, .lambda..sub.o. All other wavelengths are rejected by the filter. Filters are usually not physically coupled to the sensor. A good example of a selective filter is the dye layers used in photography, color xerography, and color printing. For example, a yellow color filter when used in photographic applications must be able to transmit yellow light very well. During its operation the filter must be placed in front of the sensor (or film) in a manner whereby all light must go through the filter in order to reach the sensor (or film). If these conditions are met, the sensor (or film) cannot "see" any other colors such as cyan or magenta. This is how effective color separation is accomplished in photographic and xerographic processes.
The use of a transmission dye layer as a wavelength selective filter for a photo-conductive device is also known in the art. Fukaya al (U.S. Pat. No. 4,700,080) is directed to the fabrication and operation of arrays of color photosensors of amorphous Si (H, X) . Fukaya et al shows that their device consists of seven layers (top to substrate):
(1) At least one color pigment layer (filter), over a PA1 (2) Protective layer (insulating organic film or resin), over a PA1 (3) Conductive layer (electrode), over an PA1 (4) n.sup.+ ohmic contact layer, over an PA1 (5) Amorphous silicon over-layer (photoconductor element), over an PA1 (6) Amorphous silicon under-layer (photoconductor element), on a PA1 (7) Substrate PA1 I. Near-Infrared Absorbing Dyes PA1 III. Macrocyclic Dyes (free-base and metallated forms useable) PA1 IV. Fluorescent Dyes PA1 V. Photochromic Dyes PA1 VI. Electrochromic Dyes PA1 VII . Photoconductive Dyes PA1 VIII. Polymeric Chromophores PA1 IX. Liquid Crystalline Dyes
Fukaya et al teaches that polychromic light irradiating onto the top filter layer of the structure results in absorption of certain wavelengths of light. Those wavelengths of light absorbed most strongly by the dye layer (#1) do not reach the photoconductive material layers below (#4 and #5), and are separated by two layers, the insulating layer #2 and the electrode #3. Only those wavelengths not absorbed by the filter are transmitted through three layers and down into the photoconductor elements wherein photogenerated charge carriers are created only by those transmitted wavelength(s). The creation of such carriers lead to an increase in the measured photocurrent of the element. These events form the basis for the photodetection of the colored light according to Fukaya et al.
Although it is quite common for dye materials to exhibit very narrow absorption bands in the visible region (see FIG. 12), in order to obtain very narrow transmission bands (as required for filter technology), it is often necessary to combine multiple filter layers due to the broad transmission regions exhibited by the dyes (see FIG. 13). This can be a considerable disadvantage due to the unavoidable losses of light of the desired wavelength that occur as a side-effect when multiple filter layers are combined. Thus, a limiting characteristic of a transmission dye filter is that in order to obtain any degree of wavelength selectivity the dye layer must either be relatively thick (greater than about 100.ANG.) or multiple dyes applied over one another. However, as the dye layer becomes thicker, the dye tends to also absorb the wavelength of light whose transmission is desired. This results in a loss in sensitivity of the photo-detecting device.
Accordingly, there is a need in the art to be able to transmit specific wavelengths of light with a high degree of selectivity without the accompanying loss of sensitivity resulting from either excessively thick transmission dye layers or multiple layers of different dyes.
Wolf, in U.S. Pat. No. 3,956,727 shows a superconducting switch or bistable device comprising a superconductor positioned in a cryogenic chamber which is maintained just below the superconducting transition temperature, T.sub.c. The cryogenic chamber includes a window that is transparent in the appropriate frequency range band for passing a laser beam, which beam impinges on a portion of the superconductor. The window is spaced away from the superconductor structure, shown in the patent drawings as a classic "Josephson junction" or superconductor "weak link structure." The impingement of the laser beam of the appropriate wavelength heats the superconductive device slightly so that its resistance increases. That is, it flips out of the superconducting state into the normally conductive metallic state, which is accompanied by a large increase in resistance. This forms the basis for the switch. Ostensibly the photo-bandpass window of Wolf is used to exclude all wavelengths of light except for the those wavelengths at or near the wavelength of the laser. Here, the light filter is used to allow only the intense monochromatic laser light to pass through while minimizing the transmission of ambient light.
Wolf and Forrester illustrate how the transmitted light in a superconducting photo-detector affects the conductivity of the superconductor. The incident wavelength of light heats up the superconductor which is maintained just below its critical temperature (T.sub.c). Depending upon the thickness of the superconductor and the amount of transmitted light allowed through the transmission filter the bolometric response of the photo-detection device can be relatively slow when compared to devices such as those described in Murakami which operate nonbolometrically.
Accordingly, there is a need for a photo-detection device that exhibits a fast response time when light of the desired wavelength is detected.
Mizutani et al in U.S. Pat. No. 4,367,369 shows a solar battery formed by using a specific merocyanine dye on an n-type semiconductor substrate. The dye functions as a p-type semiconductor, thus forming a p-n junction on the inner face between the dye film and the n-type semiconductor substrate. In addition, light permeable metallic layers are coated onto the dye to form a 4-layer laminated structure. The dye film is said to absorb visible light "in a wide range of wave lengths."
Accordingly, there is a need in the art to provide sensors and devices that are rapidly and reversibly switchable, tuneable, highly efficient, and permit direct energy transfer in an optoelectronic system that in an array permits multiplexing by discrimination of unique tight bandwidth signals from amongst multiple signals delivered synchronously or asynchronously.